There is and has been increasing use of metal containers for carbonated beverages such as soft drinks and beer. One of the earlier and most common of these containers employed an easy open closure wherein a metal ring attached to the end wall at a portion thereof which had weakened tear lines was easily lifted and pulled away therefrom, removing the portion of the end wall defined by the tear lines. Such closures have become ecologically unacceptable because the removed metal tabs are commonly discarded on the ground or in lakes. Such tabs decompose extremely slowly and, because of their sharp edges, can be dangerous.
A more recent development again utilizes weakened tear lines in the end portion of a container, but is designed to retain the closure with the container. In this instance, the metal tab remains with the end portion of the can itself after manually opening. However, any dirt and/or debris which is contained on the can end is pushed into the container during opening, because the portion of the end wall defined by the tear lines is in fact inserted into the can during opening, thus contacting the contents thereof.
It is known that unpressurized cans containing items such as tomato juice, orange juice and similar liquids can be provided with easy open closures utilizing pressure-sensitive adhesive tape tabs, such as are disclosed in U.S. Pat. No. 3,389,827. However, such closure systems have not been found acceptable for use in connection with containers in which are packaged gas-containing liquids, such as carbonated soft drinks and beer. The pressure buildup in such systems can ordinarily cause tape closures to bulge upward and gradually peel away from the area immediately adjacent the preformed opening or pour hole, whereby the can seal becomes broken.
Furthermore, when utilizing containers which enclose lubricious materials, such as motor oil, the extreme variation in temperature under which such containers are stored and used has presented significant problems when attempting to utilize easy open closure systems.
A number of patents purport to disclose various easy open closure systems which are asserted to be useful in conjunction with containers having carbonated beverages therein. Examples include U.S. Pat. Nos. 2,870,935, 3,292,828 and 3,339,788. An improvement patent over these earlier systems is U.S. Pat. No. 3,990,603, but in all of the foregoing cases, the easy open systems disclosed therein have met with only limited commercial success.
One major problem encountered in the development of such closures is the utilization of materials having sufficient physical properties to form a seal under the conditions encountered in the filling and closing of metal containers. With any of these prior closures, especially when applications involving carbonated beverages, it has been difficult to obtain an adhesive capable of meeting the stringent requirements of ready application to a container end, retention of the elevated pressure within the container, and easy opening characteristics.
We have now discovered an adhesive system which is capable of eliminating the foregoing deficiencies and providing an easy open closure system which is functional both for containers containing carbonated liquids and those containing lubricious materials such as motor oils, which of necessity must be operable over wide temperature variations.